The invention relates to a photovoltaic device for direct conversion of solar energy into electrical energy, which comprises a concentrator lens system and at least one solar cell. Furthermore the invention relates to a method for producing a concentrator lens system which can be used in a corresponding photovoltaic device but also in other devices for, e.g. thermal, use of radiation.
In the case of solar energy production with focusing lens systems, the solar radiation is concentrated, as a result of which a more efficient and more economical conversion of the solar energy into electrical energy is possible. An example of this is the FLATCON® system developed at the Fraunhofer ISE (A. W. Bett, H. Lerchenmüller, “The FLATCON® System from concentrix solar”, in: A. Luque et al.: “Concentrator photovoltaics”, 2007, pp. 301-319). The FLATCON® system is based on the fact that lens plates in which concentrating structures in the form of a large quantity of square point-focusing Fresnel lenses are applied on glass in a thin layer made of transparent silicone are used here. For this purpose, an initially liquid silicone is cured between a glass plate and a corresponding form tool which carries the desired structure of the Fresnel lenses as negative (subsequently termed negative mould) and the form tool is subsequently removed. In the FLATCON® system, lens plates produced in this way are fitted at the spacing of the focal distance of the Fresnel lenses above a base plate which is fitted with solar cells. In the case of the FLATCON® system, the focal distance is approx. twice the edge length of the individual Fresnel lenses.
Silicone offers the great advantage of astonishing durability relative to solar radiation and thermal shock load. Several years of experience in this respect show that the combination of glass (as outer cover) and silicone as structure-bearing layer leads to durable lens plates which are functional over long periods of time. In addition to other factors, the durability is not least based on the fact that the cured silicone is chemically crosslinked and the finished structure consequently has very stable viscoelastic properties. In contrast to thermoplastically shaped materials, the shape will therefore not change even over fairly long periods of time due to flowing or relaxing, as a result of which the optical function would be impaired. When using silicone as material to be embossed, disadvantages with respect to curing times and the complex production process occur. This leads to the fact that, when producing large areas or in large-scale manufacturing of lens plates, a large number of form tools is required since the dwell time during the embossing is determined by the high curing time. High specific costs respectively are associated with the requirement for a large number of embossing tools, a long processing time and a complex process implementation.
A further disadvantage when using silicone is the low refractive power which is present here. Thus the refractive index of silicones is approx. n=1.41. Correspondingly, only relatively large focal distances can be produced with representative prism angles since the reflection losses increase with large prism angles and the wavelength dispersion of light of a large spectral band width increases in an undesirable manner. In particular with respect to the costs occurring during production of the module, large focal distances are however undesired so that as small focal distances as possible are sought. In the case of a similar lens size, this requires materials with a higher refractive index if the reflection losses in the outer regions of the lenses are intended to remain low.
A further advantage of a higher refractive index resides in the fact that, with the same focal distance, the refractive structures (respectively prisms viewed in a sectional plane) become flatter. This facilitates manufacture both in the production of a tool and also in replication. In addition, the so-called disturbing edge proportion, i.e. the proportion of the projected lost area due to the optically inactive side of the prisms, reduces with flattening prisms. The higher refractive index can therefore lead to a higher optical efficiency (less disturbing edge proportions, lower reflection losses in the case of large prism angles in the outer regions of the Fresnel lens, fewer structural errors due to flatter structures) and also possibly to a more economical manufacture.